Abstrict A liquid flow meter for directly measuring the velocity of a liquid
is disclosed. The liquid flow meter includes a pair transducers
arranged facing each other in a conduit through which the liquid
flows. The liquid flow meter also includes a transmitter means for
causing the transducers to simultaneously transmit an acoustic wave
packet directed for reception at the other transducer. A differential
receiver means is also included whereby the differential receiver
means has inputs each coupled to a corresponding one of the transducers
for detecting an acoustic signal received thereby and determining
a difference between the two received signals. The difference being
related to the velocity of the liquid within the conduit. The transmitter
means and the differential receiver means are each matched to the
transducers to ensure substantial reciprocity to thereby substantially
avoid phase and/or amplitude variations in the received signal.
A method for measuring the velocity of a liquid is also disclosed.
The method involves transmitting an acoustic wave packet between
a pair of transducers that face each other, detecting an acoustic
signal received by a pair of amplifiers and determining the time
difference between the two received signals where the receiver amplifiers
are matched to the transducers through resistive components.
Claims What is claimed is:
1. A liquid flow meter for directly measuring the velocity of a
liquid, said liquid flow meter including: a pair of transducers
arranged facing each other in a conduit through which said liquid
flows: transmitter means coupled to said transducers via resistive
components, said transmitter means being configured to cause said
transducers to simultaneously transmit an acoustic wave packet directed
for reception at the other said transducer; and a pair of receiver
amplifiers having inputs each coupled to a corresponding one of
said transducers and to a corresponding one of said resistive components,
said receiver amplifiers being configured to detect an acoustic
signal received by a corresponding one of said transducers and to
determine a time difference between the two received signals, said
time difference being related to the velocity of liquid within the
conduit, wherein the transmitter means and said receiver amplifiers
are each matched to said transducers, utilising said resistive components,
to ensure substantial reciprocity to thereby substantially avoid
phase and/or amplitude variations in said received signals.
2. The liquid flow meter according to claim 1 wherein said acoustic
wave packet as transmitted preferably comprises a predetermined
plurality of cycles.
3. The liquid flow meter according to claim 1 wherein said receiver
amplifiers are configured to output the respective inputs of a differential
detector.
4. The liquid flow meter according to claim 3 wherein said differential
detector outputs a time difference waveform, wherein said time difference
waveform is related to the velocity of the liquid within the conduit.
5. The liquid flow meter according to claim 4 further including
a processing means, wherein said processing means removes noise
from said difference waveform and calculates a time difference between
said two received signals.
6. The liquid flow meter according to claim 5 said processing
means further produces a sinusoidal pulse train at a predetermined
frequency which is used to electrically excite the liquid flow meter.
7. The liquid flow meter according to claim 6 wherein said predetermined
frequency is 1 MHz.
8. The liquid flow meter according to any one of claims 3 wherein
said differential detector is formed by a transformer having terminals
of a primary winding coupled to respective outputs of said receiver
amplifiers.
9. The liquid flow meter according to claim 1 wherein said passive
ciruits are impedance matched.
10. The liquid flow meter according to claim 1 wherein said impedances
are matched during both emission of said transmit signals and reception
of said acoustic signals.
11. A method for measuring the velocity of a liquid, said method
comprising the steps of: simultaneously transmitting an acoustic
wave packet between a pair of transducers arranged facing each other
in a conduit through which said liquid flows; detecting a acoustic
signal received by each of a pair of receiver amplifiers having
corresponding first and second input nodes, each said node being
coupled to an output of said transmit amplifier stage via one of
a pair of passive circuit, and being further connected to a corresponding
one of said; and determining a time difference between the received
acoustic signals, said time difference being related to the velocity
of said liquid within said conduit, wherein an impedance formed
at one said node by an input impedance of a corresponding receiver
amplifier and the corresponding one of said passive circuits is
matched to the impedance at the other said node formed by The other
receiver amplifier and other said passive circuit.
12. The method according to claim 9 further comprising the step
of outputting a difference waveform, said difference waveform being
related to the velocity of the liquid within the conduit.
13. The method according to claim 12 including the further steps
of: removing noise from said difference waveform; and calculating
a time difference between said two received signals, utilising said
receiver amplifiers, wherein said receiver amplifiers are configured
to output the respective inputs of a differential detector.
14. The method according to claim 9 wherein a sinusoidal pulse
rain is produced at a predetermined frequency.
15. The method according to claim 11 wherein said passive circuits
are impedance matched.
16. The method according to claim 11 wherein said impedances are
matched during both emission of said transmit signals and reception
of said acoustic signals.
17. A circuit adapted for use with a liquid flow meter, said circuit
comprising: a transmit amplifier stage having an output; first and
second receive amplifier stages having corresponding first and second
input nodes, each said node being connected to said output of said
tat amplifier stage via one of a pair of passive circuits so as
to cause a transmit signal emitted from the transmit amplifier stage
to be passed simultaneously each said node; and a pair of transducers
each coupled to one of said nodes, the pairs being configured to
convey acoustic signals therebetween upon exitation by said transmit
signal, wherein an impedance formed at one said node by an input
impedance of said corresponding receive amplifier stage and the
corresponding one of said passive circuits is matched to the impedance
at the other said node formed by the other receive amplifier stage
and other said passive circuit.
18. A circuit according to clam 17 wherein said passive circuits
are impedance matched.
19. A circuit according to claim 17 wherein said impedances are
matched during both emission of said transmit signals and reception
of said acoustic signals.
Description FIELD OF THE PRESENT INVENTION
The present invention relates broadly to an electronic liquid flow
meter and, in particular to a liquid flow meter for domestic and
commercial use.
BACKGROUND TO THE INVENTION
Traditional liquid flow meters in domestic and commercial use generally
include some mechanical arrangement such as a bellows, or a vane
or impeller which actuates a totalising mechanism. Generally, a
mechanical dial arrangement indicates the total volume of liquid
that has passed through a meter Such mechanical arrangements are
not highly accurate, especially at low flow rates. Accordingly,
inaccuracies in low flow rate measurement can represent a substantial
loss of income to the supplier of water or other liquids such as
hydrocarbons.
Over recent years, there have been a number of proposals that utilise
electronics technology so as to provide for substantial higher accuracy
of the fluid flow measurement. Such systems generally incorporate
ultrasonic transducers that transmit ultrasonic signals both upstream
and downstream to measure the times of flight of the signals from
which the relative speed of the fluid can be calculated. A further
method measures the phase change between two signals which are simultaneously
transmitted from the transducers in order to calculate the speed
of the fluid.
However, problems arise with the use of ultrasonic signals due
to substantial variations in amplitude and phase of the waves transmitted
and received by the ultrasonic transducer, resulting in inaccurate
measurements. These variations can arise due to changes in temperature,
a build-up of material on the transducer heads which affects impedance
matching of circuit components, and also ageing and micro-cracking
of the transduction elements.
Variations must be allowed for during the design and calibration
of transducer circuits with the "reciprocity theorem"
being applied to the circuit components. The Chambers Dictionary
of Science & Technology (1991) defines the "reciprocity
theorem" as "the interchange of electronic force at any
one point in a network and the current produced at any other point
results in the same current for the same electromotive force".
In application to acoustics, the theorem essentially says that a
transmitter and a receiver may be swapped to give a reciprocal electro-acoustic
transformation Existing arrangements fail to achieve true reciprocity
in ultrasonic transducer calibrations. Thus, accurate measurement,
particularly at very low flow rates has been unable to be achieved.
It is an object of the present invention to substantially overcome,
or ameliorate, one or more of the deficiencies of the above mentioned
arrangement by provision of a liquid flow meter that is accurate
over a wide range of temperatures and operating conditions.
SUMMARY OF THE INVENTION
According to one aspect of the present invention there is disclosed
a liquid flow meter for directly measuring the velocity of a liquid,
said liquid flow meter including: a pair of transducers arranged
facing each other in a conduit through which the liquid flows; transmitter
means for causing the transducers to simultaneously transmit an
acoustic wave packet directed for reception at the other said transducer;
differential receiver means having inputs each coupled to a corresponding
one of said transducers for detecting an acoustic signal received
thereby and determining a difference between the two received signals,
said difference being related to the velocity of liquid within the
conduit, wherein the transmitter means and said differential receiver
means are each matched to said transducers to ensure substantial
reciprocity to thereby substantially avoid phase and/or amplitude
variations in said received signals.
The acoustic wave packet as transmitted preferably comprises a
predetermined plurality of cycles.
Preferably the differential receiver means comprises a pair of
receiver amplifiers each coupled to a corresponding one of the transducers
and outputting the respective inputs of a differential detector.
In a specific embodiment the differential detector is formed by
a transformer having the terminals of a primary winding coupled
to the respective outputs of the receiver amplifiers.
The differential detector preferably outputs a difference waveform,
wherein the difference waveform is related to the velocity of the
liquid within the conduit.
The liquid flow meter preferably includes a processing means, wherein
the processing means removes noise from the difference waveform
and calculates the difference between the two received signals.
The processing means farther preferably produces a sinusoidal pulse
train at a predetermined frequency which is used to electrically
excite the liquid flow meter. Preferably the predetermined frequency
is about 1 MHz.
According to another aspect of the present invention there is provided
a A method for directly measuring the velocity of a liquid, said
method comprising the steps of: simultaneously transmitting an acoustic
wave packet between a pair of transducers arranged facing each other
in a conduit through which said liquid flows; a detecting an acoustic
signal received by differential receiver means having inputs each
coupled to a corresponding one of said transducers; and determining
a difference between the two received signals, said difference being
related to the velocity of liquid within the conduit, wherein the
transmitter means and said differential receiver means are each
matched to said transducers to ensure substantial reciprocity to
thereby substantially avoid phase and/or amplitude variations in
said received signals.
BRIEF DESCRIPTION OF DRAWINGS
A number of embodiments of the present invention will now be described
with reference to the accompanying drawings in which:
FIG. 1 is a schematic block diagram of the liquid flow meter of
the preferred embodiment;
FIG. 2 is a schematic block diagram of the liquid flow meter electronics
circuit of FIG. 1;
FIG. 3 is a drawing showing the two received transducer waveforms
and the resulting differential waveform;
FIG. 4 is a schematic circuit diagram of one implementation of
the electronics circuit of FIG. 2; and
FIG. 5 is a detailed circuit diagram of the electronics circuit
of FIG. 4.
DETAILED DESCRIPTION OF THE DRAWINGS
The preferred embodiment is a liquid flow meter that directly measures
the velocity of a liquid by determining the time difference in reception
of two separately but simultaneously transmitted bursts of ultrasound
in opposite directions in the same tube. Ultrasonic transducers
are arranged within respective aerodynamic housings at each end
of the tube and can function as either ultrasound emitters or detectors.
In the preferred embodiment, the transducers are spaced apart by
a distance of 200 mm.
A suitable type of transducer that can be used with the preferred
embodiment is a PZT ceramic transducer (manufactured by KB-Aerotech).
The electronic liquid flow meter is provided with an electronics
circuit which generates, detects and calculates a time difference
between the two ultrasonic transducer waves,
FIG. 1 shows an overall block diagram of the liquid flow meter
1 of a first embodiment which includes two transducers 10 and 20
which are connected to an electronic circuit 30 which drives the
transducers 10 and 20 simultaneously and outputs signals 3 5 and
7 having amplitudes or phases related to the velocity of the liquid.
The output of the electronics circuit 30 is fed into an analogue
to digital converter (ADC) 40 for converting the signals 3 5 and
7 into digital form for processing by a digital signal processor
(DSP) 50. The DSP 50 performs a linear least squares fit on the
signals 3 5 and 7 to remove noise therefrom to enable the actual
flow rate to be calculated and displayed (not shown but known in
the art). The DSP 50 outputs a digital sinusoidal pulse train signal
to a digital to analogue converter (DAC) 60 which feeds back into
the electronic circuit 30 in order to drive the electronic circuit
30.
FIG. 2 shows an overall block diagram of the liquid flow meter
electronic circuit 30 of a first embodiment which includes an input
amplifier stage 70 connected to a matched resistor stage 80. The
two transducers 10 and 20 are also connected to the matched resistor
stage 80. A transducer receiving amplifier stage 90 is connected
to the two transducers 10 and 20 and feeds into a differential output
amplifier stage 100. The transducer receiving amplifier stage 90
is also separately connected to two output amplifier stages 110
and 120.
A sinusoidal pulse train at a preferred frequency of 1 MHz is applied
to the burst input 2 of the input amplifier stage 70 for a predetermined
period, typically 20 cycles. This preferred number of cycles is
long enough to allow the transducer signals to settle at a constant
amplitude and yet, short enough so that reverberation between the
transducers does not occur. In the preferred embodiment this sinusoidal
signal is supplied from the DSP 50 through the DAC 60. The signal
is buffered by the input amplifier stage 70 and output simultaneously,
via a matched resistor stage 80 to the two ultrasonic transducers
10 and 20. The transducers 1020 chosen for their stable characteristics,
send ultrasonic signals in opposing directions, up and down a moving
column of fluid. A time delay will be endured by the signal travelling
upstream in comparison to that travelling downstream. Each transducer
10 and 20 is configured to receive the ultrasonic waveform sent
by the opposing transducer and convert the respective received waveform
into an equivalent electrical waveform. This signal is then fed
into and buffered by the transducer receiving amplifier stage 90.
The outputs of the transducer receiving amplifier stage 90 feed
into the differential output amplifier stage 100 which measures
the difference of the two applied waveforms to create a difference
signal. The differential output amplifier stage 100 then amplifies
the difference signal and buffers to the output. The two further
amplifier stages 110 and 120 buffer copies of each of the received
signals of the transducers and output the signals so that the raw
signal amplitudes can be measured.
FIG. 3 shows a graph of the transducers received signals. The signal
transmitted by transducer 10 is illustrated in FIG. 3 as the downstream
signal 3. A time delay can be observed in the upstream signal 5
which was transmitted by transducer 20. The differencing operation
of differential output amplifier stage 100 generates a signal 7
which is illustrated in the bottom graph of FIG. 3. The signal 7
has an amplitude that is simply related to the time difference in
reception of the simultaneously transmitted downstream signal 3
and upstream signal 5.
FIG. 4 shows a circuit diagram of the liquid flow meter electronic
circuit 30 of the first embodiment. The input amplifier stage 70
includes an operational amplifier (op amp) IC1 configured in a non-inverting
amplifier configuration using negative feedback resistor R3 via
lines 21 and 22 and a bootstrapped input, via components R2C28.
Op amp IC1 is preferably chosen and configured to have a high input
impedance and a very low output impedance which is in the range
of milliohms. The high input impedance effectively decouples the
signal input from the circuit. This is supplemented by the bootstrap
connection mentioned above. R2 and C28 have a sufficient time constant
such that the voltage at the negative input is equal to the voltage
at the positive input, and therefore the current through R2 and
C28 is nominally equal to zero. The capacitor C28 providing further
blocking at lower input frequencies.
The output of the input amplifier stage 70 is simultaneously applied
to two matched resistors R.sub.A and R.sub.B, to the ultrasonic
transducers 10 and 20. The matching of the two resistors is critical
so that reciprocity holds.
The two transducers 10 and 20 are connected to two separate receiving
amplifiers IC4 (via line 23) and IC5 (via line 24), respectively,
which embody the transducer receiving amplifier stage 90.
In the case of transducer 10 upon receiving the transmitted signal
from transducer 20 the received signal is converted from an ultrasonic
to an electrical signal and applied to op amp IC4. Op amp IC4 is
configured as a unity gain voltage follower. The op amp IC4 being
chosen to have a high input impedance, a stable response and low
noise. The capacitance C14 on the output of IC4 is chosen large
enough to act as a short circuit at the preferred frequency. The
value of resistance R12 is nominal and does not affect the signal.
This circuit is mirrored in the case of transducer 20 for op amp
IC5. Both of these amplifiers differentially drive a floating winding
W1 of a transformer T1 (via lines 25 and 26), with a secondary winding
W2 which provides an input with respect to ground to differential
output amplifier stage 100 the latter being formed using an op
amp IC7 configured in a non-inverting manner. A resistor R19 is
connected across the secondary winding W2 of the transformer T1
and provides a current path for the transformer T1 secondary induced
e.m.f. current, thereby providing a voltage signal to the positive
input of op amp IC7 via line 27. The secondary winding W2 of transformer
T1 thereby registers the difference of the two applied waveforms
which were input to either leg of the transformer T1 primary. Therefore,
if both applied waveforms are in phase and of the same amplitude,
there will be no induced e.m.f. current in the secondary winding
W2 of transformer T1 and therefore no voltage input to the op amp
IC7.
The difference signal, registered by the transformer T1 secondary
is amplified by op amp IC7 and buffered for output through resistor
R18. The transformer T1 and op amp IC7 being chosen to have a high
common mode rejection ratio so as to reduce any noise associated
with the differencing operation of transformer T1.
The two amplifiers IC4 and IC5 which make-up the transducer receiving
amplifier stage 90 individually feed into two further separate
output amplifier stages 110 and 120. The output of op amp IC4 feeds
directly, via line 28 into the positive input of op amp IC6 which
is configured as a unity-gain voltage follower. The op amp IC6 buffers
the transducer 10 received signal and outputs a copy of the received
signal, via resistors R10 and R16. The arrangement of output amplifier
stage 110 which includes op amp IC6 is mirrored for output amplifier
stage 120 with op amp IC5 feeding directly, via line 29 into op
amp IC8. Op amp IC8 is also configured as a unity-gain voltage follower.
The differencing operation of the differential output amplifier
stage 100 generates an output signal 7 with an amplitude that is
related to the time difference upon reception between the two received
transducer signals 35. This output signal 7 is measured by an analog-to-digital
converter 40 in the preferred embodiment as seen in FIG. 1. In
a further embodiment, an oscilloscope may also be used to measure
the output. The output is finally sent to a digital signal processor
50. A least-squares fit is carried out on the differential output
signal to further reduce the noise in the estimate of its amplitude.
This gives a more accurate estimate of the time difference between
the two received signals. Once the time difference has been calculated
by the computer, the data can be inverted to give an estimate of
the fluid flow rate.
Since a change in the amplitude of the received raw transducer
signals 35 will affect the amplitude of the difference signal as
well, any change in the raw signals needs to be counteracted. This
is achieved in the preferred embodiment by measuring any changes
in the raw signal amplitudes and then dividing the differential
output signal by the secondary measured amplitude.
FIG. 5 shows a detailed circuit diagram of the liquid flow meter
electronic circuit 30 of the first embodiment, which shows all component
values and IC numbers. The positive and negative power supply terminals
of IC1 are tied to the +5 Vdc and the -5 Vdc regulated power supply
rails, respectively. Capacitors C1 C2 C5 C3 and C4 provide filtering
and aid in regulating the voltage on the positive and negative supply
rails. A similar power supply and filtering arrangement is connected
to the other op amps IC4. IC5 IC6 IC7 and IC8. IC1 is an OPA621
chip (manufactured by Precision Monolithics) which has been chosen
and configured to have a high input impedance and a very low output
impedance. The values of R.sub.2 =100.OMEGA. and C.sub.28 =100 pf
give a time constant of 100 .mu.s and therefore the circuit input
current is nominally equal to zero.
The output impedance of IC1 is in the milliohm range and therefore
the signal seen by the matched resistors R.sub.A,R.sub.B is nominally
identical to the input signal with the amplifier providing good
current drive.
The matched resistor stage is formed of two 50 ohm matched resistors
R.sub.A, R.sub.B. The resistor R.sub.A is configured as the parallel
combination of R.sub.4 =100 ohms, R.sub.5 =100 ohms and R.sub.6.
R6 is adjusted to match the two impedances. Typically a small outlying
transistor (50T) or surface mount resistor is used. This arrangement
is mirrored in the parallel combination of R.sub.7 =100 ohms, R.sub.8
=100 ohms and R.sub.9 for resistor R.sub.B. The resistors R.sub.4
R.sub.5 R.sub.7 and R.sub.8 being chosen to have tolerance values
of .+-.0.1%. Critical matching can be carried out for both parallel
combinations through the adjustment of R.sub.6 and R.sub.9.
Op amp IC4 and IC5 are both AD829 chips (manufactured by Analog
Devices) and were chosen to have a high input impedance and good
phase and amplitude stability. The high input impedance further
improves the matching of the impedance seen by the two transducers
10 and 20 minimises loading of the input driving stage and ensures
reciprocity. The phase and amplitude stability ensure that the received
signals applied to the transformer T1 primary winding W1 are identical
to those seen by the transducers 10 and 20. Capacitor C15 is used
for bandwidth compensation. The capacitors C14 and C21 are both
equal to 100 nf and will act as a short circuit at the preferred
operating frequency of 1 MHz. The resistance R.sub.12 =100 ohms
is a nominal value and will not affect the signal. This value of
R.sub.12 is chosen in order to minimise noise and to provide good
bandwidth response The power supply configuration, bandwidth compensation
and output configuration of IC4 is substantially mirrored for IC5.
Transformer T1 is a Philips 3C85 core transformer and has been
chosen for its high CMRR. The transformer has a turns ratio at 4:16
and therefore provides amplification of the difference waveform
which is applied to its primary.
Op amp IC7 is an AD829 chip. Resistors R.sub.11 =2 k49 ohms and
R.sub.13 =105 ohms provide a gain of approximately 25 for op amp
IC7. Op amp IC7 amplifies the difference waveform and buffers it
for output thereby decoupling the output stage. Compensation capacitor
C30 sets the bandwidth response for op amp IC7 at a predetermined
value.
Op amp IC6 and IC8 are also AD829 chips being chosen for their
high input impedance and low output impedance characteristic, decouple
the circuit from the output and provide good current drive. Compensation
capacitor C29 is used to adjust the bandwidth response of operational
amplifier IC6. The power supply configuration and bandwidth compensation
of IC6 has essentially been mirrored for IC8.
FIG. 5 also shows two voltage regulator circuits which supply the
+5 Vdc and -5 Vdc power supply rails for the operational amplifier
stages. Voltage regulator IC2 has a +12 Vdc input to provide a +5
Vdc output. Capacitors C6 C8 and C9 C7 provide input and output
filtering, respectively, for the voltage regulator IC2 and regulate
the output voltage at the desired +5 Vdc level. This circuit is
mirrored in the case of the -5 Vdc rail, where voltage regulator
IC3 is provided with filtering capacitors C10 to C12.
The above-described embodiment has several advantages which are
outlined as follows:
First, due to the very low output impedance of IC1 the very high
input impedance of IC4 and IC5 and the critical matching of the
two 50 ohm transducer input resistors, true reciprocity is achieved
in the preferred embodiment. As a result, the measurements are less
susceptible to drift caused by changes in the temperature of the
transducers.
Second, the two output amplifier stages 110 and 120 further reduce
inaccuracies in the calculated difference signal, by providing the
received raw signal amplitudes to be measured. These measurements
are then taken into account when calculating the final output difference
amplitude and counter-act any discrepancies in the transmitted transducer
signal.
The foregoing describes only one embodiment of the present invention,
and modifications, obvious to those skilled in the art, can be made
thereto without departing from the scope of the present invention. |